(a) Field of the Invention
The present invention relates to a retardation film including: a polymer resin; and an optically anisotropic compound having a silicon group, and a polarizing plate including the retardation film. The present invention also relates to a liquid crystal display including the retardation film and/or the polarizing plate.
(b) Description of the Related Art
As a sharper image has been required with the recent expansion of the display market, an optical material having an advanced optical characteristic, instead of a simple transparent material, has been required. Especially, since a liquid crystal display (LCD) used for notebooks, monitors, and TVs, as compared to a CRT (cathode-ray tube) and a PDP, has a poor viewing angle characteristic, various investigations for making up such a characteristic have been conducted.
As one method of improving a viewing angle characteristic, there is a method of employing, as a retardation film, a stretched film obtained through uniaxial stretching or biaxial stretching of a transparent film including a polycarbonate, a cellulose resin, or a cyclic olefin-based resin.
In the case of a polycarbonate, a relatively large phase difference can be shown by stretching, but due to a high elastic modulus at light, the phase difference is subject to change according to a stretching process condition or use environment. In addition, unevenness of phase difference may occur.
Meanwhile, a cyclic olefin-based resin, or a cellulose acetate-based resin can show very high film transparency, but shows a small phase difference through a stretching process. Also, in order to obtain a large phase difference, it is necessary to stretch a film to a great extent, which may cause unevenness of phase difference.
Japanese Patent Laid-Open No. 2004-035347 has recently disclosed a method of fabricating a retardation film by introducing a needle-shaped inorganic particle to a cyclic olefin film. However, in the case of an inorganic fine particle, it is difficult to uniformly disperse the particle in a solvent due to low solubility in the solvent, and high density, and there is a possibility that the particle is subject to aggregation due to low stability, thereby reducing the transparency of a polymer film.
Also, Japanese Patent Laid-Open No. 2006-154760 disclosed a retardation film obtained by adding a low-molecular weight organic compound within a transparent polymer film, and carrying out a stretching process. However, in this invention, the organic compound has low anisotropy, and stretching of the polymer film results in only a limited phase difference. Thus, in order to obtain a required phase difference, it is necessary to stretch a film to a great extent.
Also, in general, a compound with high anisotropy has a high degree of crystallinity, low solubility in a solvent, and low compatibility within a polymer film. Thus, when mixed and processed, such a compound is deposited/phase-separated and thereby is difficult to fabricate a transparent film. However, no solution has yet been proposed.